Electronics > Repair

HP 6114A Newbie Repair

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hifiguy99:
Hi all,
I have an old HP 6114A. I have a few things I noticed that are wrong. I am inexperienced with vintage electronics and any input I would appreciate.
I have an imgur album here: https://imgur.com/a/RlH44oH
It seems I can't upload the pictures here directly as they are too big.

It buzzes quite loudly from the transformer. I assume this is typical of equipment this old, correct?.

The PCB is black. I mean not the entire PCB but I have no idea what would cause this exactly. The top underside panel is darkened in two spots. In does release some heat. Is this a major issue? See the picture:

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You can see the distinction here:

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As for any of the capacitors, should I replace any of them? I know tantalums go bad with age. I assume they are the yellow ones axial ones? Should I just leave it till one goes? None of the caps on the top side or mains side seem budging:

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Lastly, if I halfway press the 2nd digit on the decade switch or if I switch voltages around too quickly, it jumps to a random voltage different than what is selected - usually higher. The meter shows it accurately though. Sometimes when normally switching voltages it will spike instantly and go back to normal.
Would a cleaning of the decade switch usually fix this? Example:

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Thanks!

bob91343:
That 10W resistor is clearly too small.  It's probably dissipating several Watts, like around 100 mA through it for nearly 80V drop.  Its paint job has boiled off onto the cover.  Look in the manual to see what the voltage across it should be and that will tell whether it is a bad design or a problem elsewhere.

As for the switch problem, it does appear a cleaning is in order.

The transformer buzz is due either to the overstress implied above, or that the bolts holding the laminations together have loosened.

wn1fju:
Your board doesn't look too bad to me.  I've attached a picture of my closely related HP 6115A power supply.  I wouldn't start replacing capacitors unless one (or more) is obviously spewing out electrolyte or voltage measurements show something is wrong.

You will need to remove the digit switches, dissect, clean, and reinstall.  See a pictorial in the eevBlog post:

https://www.eevblog.com/forum/testgear/hp-6114a-precision-power-supply-restoration/

garrettm:
I would use an oscilloscope to check that the programming switches aren't causing large spikes / drops in the output voltage. One unit I had would jump around wildly until I cleaned the switches. Turns out the lubricant had dried up and was causing problems with the rotary contacts mating with the PCB pads when setting the voltage.

Also, you should clean the slide switch contacts, my analog display was acting goofy until those were properly cleaned too.

I've owned three of these power supplies, one of which was silent, the other two had noticeable transformer hum but one was much louder than the other. So the loud hum might just be an unlucky unit rather than indicating something is wrong.

I've made a imgur gallery with photos from my repair. Hopefully they can help you with yours.

https://imgur.com/a/sf8hrbu

hifiguy99:

--- Quote from: bob91343 on January 29, 2021, 07:13:19 am ---That 10W resistor is clearly too small.  It's probably dissipating several Watts, like around 100 mA through it for nearly 80V drop.  Its paint job has boiled off onto the cover.  Look in the manual to see what the voltage across it should be and that will tell whether it is a bad design or a problem elsewhere.

As for the switch problem, it does appear a cleaning is in order.

The transformer buzz is due either to the overstress implied above, or that the bolts holding the laminations together have loosened.

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Your are correct in that the resistor is small. I am lazy though and just cleaned off the paint off the surrounding components with some isopropyl alcohol and a Q-tip. The Q-tip could not remove the black paint caked on the motherboard though. I didn't try other solvents. Thanks anyways.
Tightening the bolts that hold the transformer do not help unfortunately.


--- Quote from: wn1fju on January 29, 2021, 12:38:00 pm ---Your board doesn't look too bad to me.  I've attached a picture of my closely related HP 6115A power supply.  I wouldn't start replacing capacitors unless one (or more) is obviously spewing out electrolyte or voltage measurements show something is wrong.

You will need to remove the digit switches, dissect, clean, and reinstall.  See a pictorial in the eevBlog post:

https://www.eevblog.com/forum/testgear/hp-6114a-precision-power-supply-restoration/

--- End quote ---

I dissected the mechanical decade switch and used dexoIT gold on the contact plate and some Non-Stick Dry Film lubricant (with teflon) for the innards. It works like a charm now. Thanks for the help. 


--- Quote from: garrettm on January 31, 2021, 02:22:21 am ---I would use an oscilloscope to check that the programming switches aren't causing large spikes / drops in the output voltage. One unit I had would jump around wildly until I cleaned the switches. Turns out the lubricant had dried up and was causing problems with the rotary contacts mating with the PCB pads when setting the voltage.

Also, you should clean the slide switch contacts, my analog display was acting goofy until those were properly cleaned too.

I've owned three of these power supplies, one of which was silent, the other two had noticeable transformer hum but one was much louder than the other. So the loud hum might just be an unlucky unit rather than indicating something is wrong.

I've made a imgur gallery with photos from my repair. Hopefully they can help you with yours.

https://imgur.com/a/sf8hrbu

--- End quote ---

Good to know the hum is normalish. Thanks for the photos. I did spray down the meter switch with some Deoxit and then again with some of the lubricant as they do tend to get gunked up over the years, although my analog display was working fine.
I checked with a scope and the voltage changes look smooth now. Thanks for the tip. 

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